Ultrahigh resolution spectroscopy

As described in H-I, ultracold hydrogen has great potential for ultrahigh resolution spectroscopy of hydrogen [16], In particular, it holds the promise of providing a resolution close to the natural linewidth, 1.3 Hz. As documented by presentations at this conference, this transition is a touchstone for spectroscopy, studies of fundamental theory and determination of fundamental constants [17]. 

Ultrahigh resolution spectroscopy requires ultrastable lasers. Fortunately, there has been major progress in this areas. A laser locked to an external reference cavity [47] has yielded a resolution of a few parts in 1015 for an Hg+ ion in a trap [48]. 

E.M. Belenov, M.V Danileiko, V.R. Kozuborskh, A.P. Nedavnii, M.T. Shpak, Ultrahigh resolution spectroscopy based on wave competition in a ring laser. Sov. Phys. JETP 44,40 (1976)... 

For some tasks in ultrahigh-resolution spectroscopy, the residual finite linewidth AyL, which may be small but nonzero, still plays an important role and must therefore be known. Furthermore, the question why there is an ultimate lower limit for the linewidth of a laser is of fundamental interest, since this leads to basic problems of the nature of electromagnetic waves. Any fluctuation of amplitude, phase, or frequency of our monochromatic wave results in a finite linewidth, as can be seen from a Fourier analysis of such a wave (see the analogous discussion in Sects. 3.1,3.2). Besides the technical noise caused by fluctuations of the product nd, there are essentially three noise sources of a fundamental nature, which cannot be eliminated, even by an ideal stabilization system. These noise sources are, to a different degree, responsible for the residual linewidth of a single-mode laser. 

Macfarlane R.M., Shelby R.M., Shoemaker R.L. Ultrahigh-resolution spectroscopy Photon echoes in YA103 Pr and LaF3 Pr +. Phys. Rev. Lett. 1979 43 1726-1730... 

For some tasks in ultrahigh-resolution spectroscopy, the residual finite line width which may be small but nonzero, still plays an important... 

For some problems in ultrahigh-resolution spectroscopy, the residual finite linewidth Av, which may be small but nonzero, still plays an important role and has therefore to be known. Furthermore the question why there is an ultimate lower limit for the linewidth of a laser is of fundamental interest, since this leads to basic problems of the nature of electromagnetic waves. 

Figure 13.5 illustrates the narrowing of the two-photon resonance with two separated interaction fields. The technique of two-photon Ramsay resonances has been succesfully applied to the ultrahigh resolution spectroscopy of Rydberg levels in the rubidium atom using a well-stabilized cw dye laser [13.6]. At a separation of 4.2 mm between the two fields, Ramsay fringes with... 

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